MINIMIZING RISK DURING TUNNEL CONSTRUCTION BY MONITORING
Wulf Schubert
Graz University of Technology
3G-Gruppe Geotechnik Graz ZT GmbH
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INTRODUCTION
■ Risk in engineering commonly is understood as the productof probability of an event and the consequences of such an event
■ It is also understood that in case a risk has an unacceptable level, mitigation measures have to be taken to either eliminate the hazard, reduce the probability of its occurrence, or reduce the consequences
■ Uncertainties in the ground model, the physical properties of the ground, as well as the limited accuracy and simplifications in the mathematical models used for design do not allow precisely predicting the behaviour of the tunneland the surrounding ground during the design
■ Thus a residual risk remains, which has to be dealt withduring construction
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HAZARDS IN TUNNELLING
■ In tunnelling unfavourable events (hazards) can be:
□ Damage of the support due to excessive deformations or loads□ Collapses with or without damage to third parties□ Excessive ground deformations, leading to damage of structures
and utilities□ Lowering of ground water table□ Immissions by blasting vibrations, noise, dust,□ etc.
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UNPREDICTABLE ? UNAVAOIDABLE ?
REUTERS/Gilberto Marques (BRAZIL)
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■ The inherent inaccuracy of the design calls for an observational approach to allow for a safe and economical construction with minimal risk
■ A successful implementation of the observational approach requires sound preparation, including establishment of a targeted monitoring program, appropriate site organization and a safety management plan
■ Focus of observation on verification of assumptions and identification of (predicted or unpredicted) hazards
■ To be able to detect deviations from the “normal behaviour”the expected behaviour and limits of acceptable behaviourneed to be defined
OBSERVATIONAL APPROACH
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PURPOSE OF MONITORING
■ Observe system behaviour and check if it is within theacceptable range
■ Use data to verify assumptions made during design on ground behaviour
■ Calibrate models
■ Use results for fine tuning of excavation and support
■ Use data for prediction of ground structure and qualityahead of the face
■ Proof that prescribed limits are kept (for example surfacesettlements)
■ Legal aspects
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DISPLACEMENTS IN UNDERGROUND CONSTRUCTION
■ Magnitude and characteristics of deformation depend on ground quality and structure, initial stress, tunnel size, construction sequence, and support
■ The monitoring programme must fulfill followingrequirements:
□ Capture the expected behaviour□ Monitoring intervals short enough to allow untertaking sucessful
contingency actions□ Processing and analysis of data sufficiently rapid in relation to
possible evolution of the system
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MONITORING METHODS FOR TUNNELS
■ A number of methods exist to measure displacements and strains above ground and in the tunnel
■ Above ground (selection)
□ Levelling□ Inclinometers, extensometers□ Tiltmeters□ Measurement of displacements by total station
■ In tunnel (selection)
□ Measurement of displacements by total station□ Extensometers□ Strain meters□ Measuring bolts
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TYPICAL INSTRUMENTATION FOR SELECTED SECTIONS
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TYPICAL DEVELOPMENT OF DISPLACEMENTS
■ Tunnel excavation causes a change in the boundaryconditions and stresses (stress increase and relaxation)
■ This naturally leads to displacements of the ground, usuallya reduction in tunnel size
■ Influence of stress rearrangement reaches ahead of theface and is generally completed a couple of diametersbehind the face
■ Highest displacement gradient usually at the face
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TYPICAL DEVELOPMENT OF DISPLACEMENTS
-25 -20 -15 -10 -5 0 5 10 15 20 25
time / distance
disp
lace
men
t
displacement ahead of face
displacement behind face
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MATHEMATICAL DESCRIPTION OF DISPLACEMENTS
■ Sulem and Panet have developed an empirical relationshipfor the displacements behind the face in relation to face advance and time
3,02
111,tT
TmxX
XCtxC x
xC Final time independent displacement
txC , Displacement in relation to face advance and time
X, T Shape parameters
x, t Distance face-monitoring section, time since excavation
m Proportion of time dependent displacements
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INFLUENCE OF PARAMETER X
Distance to face
-80
-70
-60
-50
-40
-30
-20
-10
00 20 40 60 80 100 120 140
disp
lace
men
ts
Distance to face
-80
-70
-60
-50
-40
-30
-20
-10
00 20 40 60 80 100 120 140
disp
lace
men
ts
X= 25
X= 6
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INFLUENCE OF FOLIATION ON DISPLACEMENT DEVELOPMENT
■ When discontinuity strength is low, stress redistribution quitedifferent for both cases
distance to face (m)
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
60-40 -30 -20 -10 0 10 20 30 40 50 60
disp
lace
men
ts (m
m)
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GeoFit® – A TOOL FOR PREDICTING DISPLACEMENTS
■ The formulation from Sulem and Panet has shown to represent „normal“ development of displacements
■ Barlow and Sellner have further developed the method, now allowing for evaluation of displacements ahead of theface, as well as simulation of supports and sequentialexcavation
■ Sellner has also extended the features allowing for thesimulation of surface settlements and developed software(GeoFit®), which incorporates all the features
■ Sellner then has developed software which allowsconsidering face advance, time effects
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PREDICTION OF DISPLACEMENTS
■ Basically two options exist:
□ Determination of function parameters by automatic fitting to measured values. A minimum of 3 readings (zero reading plus two consecutive readings) is required to obtain a fit. Fit can beimproved, as more readings are available
□ Estimating funtion parameters from previous experience and make forward prediction; Prediction can be improved, as moredata are available
■ With an assumed excavation progress, „normal“development of displacements can be established and actual readings then compared to the predicteddisplacements
■ Effects of different supports can be assessed in advance
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FUNCTION PARAMETERS BY FITTING
■ Using fit function after a few readings and extrapolation of top heading displacements for assumed progress
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PREDICT DISPLACEMENTS FROM BENCH EXCAVATION
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COMPARE MEASURED TO PREDICTED
■ Progress different form prediction, but predicteddisplacements adjusted to real sequence
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FORWARD PREDICTION BY EXTRAPOLATION OF FUNCTION PARAMETERS
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PREDICTION OF DISPLACEMENTS TOP HEADING
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EFFECT OF TOP HEADING INVERT
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PREDICTION FOR BENCH AND INVERT EXCAVATION
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COMPARISON PREDICTION - MEASURED
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Failure top heading invert
collapse
DEVIATION FROM „NORMAL“ BEHAVIOUR
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SURFACE SETTLEMENTS
■ In particular in built up areas, the surface settlementsrequire special attention
□ Buildings and utilities have limited tolerance againstdeformation, in particular in case of different displacements at different locations (e.g. different settlements at the corners of buildings or along a utility line)
■ Early detection of unfavourable development thus veryimportant
■ Basically same tools as for prediction of displacements in tunnels can be used
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COMPARISON PREDICTED SURFACE SETTLEMENTS - MEASURED
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ABNORMAL BEHAVIOUR – WHY ?
MS
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SUDDEN SUBSIDENCE ON APRIL 15TH (FACE HAS PASSED FAULT)
MSFAULT
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DISCUSSION
■ For controlling the system behaviour it is important to have a model (predicted displacements in quality and quantity) and compare the monitoring data to the model
■ In case of disagreement of monitored data and model, thereis always a reason
■ Explaining the reason for unusual behaviour besidesexploiting the data in the right way requires understandingthe mechanical processes of the system
■ Using a mathematical description of the „normal“displacement development is one of the options to easilydetect abnormal behaviour
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PREDICTION OF GROUND QUALITY AHEAD
■ Most costly events are associated to „unexpected“ changes in the ground quality
■ Analysing displacement monitoring data in a non-traditionalway, allows relatively reliably predicting such changes at practically no additional costs
■ The idea: if there are no significant changes in the rock massquality or structure in the vicinity of the tunnel, thendisplacement characteristics should be similar.Vice versa, changing ground conditions should show in changed displacement characteristics
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EXAMPLE OF PREDICTING CHANGING GROUND BY EVLAUTION OF DISPLACEMENT TRENDS
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CAUSE OF CHANGING TREND: FAULT ZONE AHEAD
Fault zone
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COMBINATION OF TRENDS FOR SPATIAL ORIENTATION
Vertical displacements crown
Deviation of displacement vectororientation in longitudinal section
Deviation of crown vector from vertical(cross section)
Variation of ratio of verticaldisplacements of sidewalls
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TREND CORRELATION MATRIX
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PREDICTOR
■ Using various trends as input, the type and spatialorientation of a change in the ground conditions can beevaluated
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EVALUATING UTILIZATION OF LININGS
■ Back calculating strains from displacement monitoring and using a model for the time dependent properties of shotcrete allows evaluating the utilization of the lining at any time
■ This is important information in particular for tunnels withlow overburden, where the shotcrete lining is the majormeans of support
■ Continuous follow up of the lining utilization allows initiatingmitigation measures in time, thus reducing the risk
■ Algorithms have been implemented in advanced monitoringevaluation software (e.g. Tunnel:Monitor)
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SHOTCRETE PROPERTIES
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EXAMPLE OF UTILIZATION OF LINING
Safety management during construction
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BASICS OF SAFETY MANAGEMENT
Hypotheses:
■ What can go wrong, usually goes wrong
■ Even things happen, which nobody thoughtcould happen at all
■ Human beings are all but perfect
■ Nothing is unpredictable, but sometimes we are notsmart enough to see the signs
Thus:
■ Prepare for all possible and apparently impossible events !
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ELEMENTS OF SAFETY MANAGEMENT PLAN
■ Identification of safety relevant issues
■ Definition of expected behaviour
■ Definition of parameters to be observed, observationmethods, layout, reading frequency, and evaluation methods
■ Definition of warning and alarm levels and criteria
■ Definition of contingency measures for each warning level
■ Action plan in case of an alarm
■ Organisation plan and reporting structure
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Observations relevantfor safety
Information ofENG
DiscussionENG and GE
Warninglevel?
Behaviour as expectedContinue as planned
Criteria attention levelreached
Criteria warning level 1reached
Criteria warning level 2reached
3rd parties not afffected
Criteria warning level 3reached
3rd parties afffected
GE: daily geotech.reportmeet with ENG and COcontact with IE, D
ENG: information CM
Measures acc.to recommendationof GE, D and IE
GE: daily geotech.reportmee with ENG and COcontact with IE, D, CHE
ENG: info CM, OTmeasures acc to recommendation of GE, D and IE and ownassessment;Coordination with D, CHE
ENG: information CM, OT, CM, GE, IE, D, CHE
ENG, CO: initiatemitigation measures
CM: crisis management
ENG: inform and protectthird partiesInformation CM, OT, CM, GE, IE, D, CHE
ENG, CO: initiatemitigation measures
PO: crisis management
Management of crisisby ENG until takeoverby CM, resp. Owner
CM: calls special geotechnical meeting for fixing of further actions
Regular geotechnicalmeeting
optional
mandatory
ACTION AND REPORTING PLAN
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WARNING AND ALARM CRITERIA
■ Criteria for the single warning levels can be fixed values, combinations of values, or trends
□ Example: criteria for subsidence can be expressed in terms of absolute values of settlement or as maximum slope, or a combination of both
■ Expected development of system behaviour in relation to time oradvance should be defined to allow for timely reaction
□ Example for surface settlement values: - 10 m ahead of face: 3 mm- at face: 10 mm- 5m behind face: 20 mm- 20 m behind face: 30 mm
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CASE HISTORY
Actual developmentof displacements
Expected developmentof displacements
Procalmation of warning level 2 due tounfavorable trend of displacements;Initiation of mitigation measures
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DISPLACEMENT VECTORS CLEARLY INDICATE PROBLEM
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MITIGATION MEASURES
■ Additional bolting on a length of about 20m on the richt sidewas ordered and executed immediately
■ A set of additional measures, like installation of a temporarytop heading invert was prepared, should the initial mitigationmeasures not show satisfying effect
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EFFECT OF MITIGATION MEASURES
Additional bolting
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PREDICTION OF FURTHER DEVELOPMENT
Bench excavation
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FURTHER DEVELOPMENT
■ Initial mitigation measures effective, but expected total displacements likely to exceed deformation allowance
■ To keep within deformation tolerance, top heading invert was installed
■ This stopped deformations practically completely
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FURTHER DEVELOPMENT
Second set ofmitigation measures
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CONCLUSION
■ Risk oriented construction requires sound preparation and understanding of potential failure mechanisms
■ Potential stability problems frequently have their cause in features outside the visible area, thus using the informationfrom face mapping only is not sufficient to assess potential hazards
■ Appropriate monitoring and evaluation concept helps in identifying critical developments and significantly reduces theprobability of experiencing „surprises“ during excavation, thus saving time and money
■ Last, but not least, an efficient safety management systemcan only be established and executed by persons, whounderstand the relationships between causes and effects
53Schubert: Minimizing risk during tunnel construction by [email protected]